It is well known to use extruded tubes of polytetrafluoroethylene (PTFE) as implantable intraluminal prostheses, particularly vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatibility. PTFE tubes may be used as vascular grafts in the replacement or repair of a blood vessel as PTFE exhibits low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft.
Grafts formed of ePTFE have a fibrous state which is defined by interspaced nodes interconnected by elongated fibrils. The spaces between the node surfaces that is spanned by the fibrils is defined as the internodal distance (IND). A graft having a large IND may enhance tissue ingrowth and cell endothelization by a significant portion of the graft having interior voids some of which provide passages through the tube wall between the outer and inner surfaces thereof. This provides the graft with porosity.
Microporous ePTFE tubes for use as vascular grafts are known. The porosity of an ePTFE vascular graft may be controllably varied by controllably varying the IND. For example, an increase in the ND within a given structure may result in an increased porosity, i.e., increased pore size, by increasing the distance between nodes resulting in thinning of the fibrils. This, in turn, results in larger voids, i.e., pores, in the ePTFE material. Increased porosity typically enhances tissue ingrowth as well as cell endothelization along the inner and outer surface of the ePTFE tube.
Increasing the porosity of an ePTFE tube, however, may limit other properties of the tube. For example, increasing the porosity of the tube may reduce the overall radial and tensile strength thereof as well as reduce the ability of the graft to retain a suture placed in the tube during implantation. Such a suture typically extends through the wall of the graft. Also, such microporous tubes tend to exhibit low axial tear strength, so that a small tear or nick will tend to propagate along the length of the tube. Thus, if the ePTFE tube has a uniform porosity along its length, the degree of porosity of therein may be limited by the strength requirements of the tube.
Alternatively, if the strength requirements for the PTFE tube may be satisfied by selected longitudinal sections of the tube having the required strength, then it may be possible for other longitudinal sections of the tube to have an elevated porosity, even if such other longitudinal sections have limited strength. For example, it may be desirable for selected axial portions of the ePTFE tube to have sufficient stiffness to prevent kinking. Such stiffness may be provided by increasing the stiffness of such axial portions where such axial portions have an annular cross-section and accordingly, the shape of individual rings. Such axial portions may typically be spaced apart from one another longitudinally and nevertheless provide the necessary stiffness to the vascular graft. Therefore, the portions of the graft between stiffened axial portions may have a lower requirement for strength and may therefore have a higher porosity.
Another example of one or more selected axial portions of a vascular graft having increased requirements for strength is where such one or more portions are to be pierced for insertion of a suture therethrough. If the portion of the graft to be pierced can be identified just prior to the piercing, then other longitudinal sections of the graft may have lower strength requirements and therefore have a higher porosity.
Another possible technique for increasing the radial tensile and axial tear strength of microporous ePTFE tubes is to modify the structure of the extruded PTFE tubing during formation so that the resulting expanded tube has non-longitudinally aligned fibrils. Forming an expanded PTFE tube with non-longitudinally aligned fibrils is typically difficult as it may require extrusion of the tube using complex equipment before expansion of the tube. Other possible methods for forming non-longitudinally aligned fibrils would be expected to be complex.
Additional properties, which may or may not be related to porosity and strength, may be desirably varied along the length of a ePTFE tube. For example, it may be desirable for the density of the tube to vary for different longitudinal positions on the tube. Density may be related to porosity, e.g., inversely proportional thereto, since the greater the voids in a selected section of the ePTFE tube, the lower the weight of the section.
Another property of an ePTFE tube which may desirably be varied along the length thereof is the number and thickness of the fibrils connecting individual nodes. This property may also be related to porosity because increasing the number and thickness of such fibrils may reduce the size of the voids in the ePTFE tube and thereby reduce the porosity thereof. Accordingly, if the number and thickness of fibrils is not reduced by an increase in the IND, then such an increase may not result in an increased porosity of the ePTFE tube.
A further property of an ePTFE tube which may be desirably varied along the length thereof is the length of the fibrils connecting individual nodes. Increasing the length of the fibrils increases the flexibility of the ePTFE tube, even if the number and thickness of the fibrils is not changed.
A possible technique for varying the properties of an ePTFE along the length thereof may include longitudinally expanding the entire tube and then longitudinally compressing selected axial portions thereof. Such longitudinal compression typically results in a decrease in the IND, and decreased porosity in the compressed axial portions. Also, such longitudinal compression typically results in bending or folding of the fibrils. Also, the microstructure resulting therefrom differs significantly from the node and fibril microstructure which would be caused by longitudinal expansion of the axial portion to the same axial dimension as results from the longitudinal compression.